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Fatima Eddoumy
To cite this version:
Fatima Eddoumy. Impact of texturing on sliding wear behaviour of UHMWPE. Mechanics of materials [physics.class-ph]. Université de Strasbourg, 2012. English. �NNT : 2012STRAD011�. �tel-00757633�
- Centre de Recherche Public Henri Tudor, Advanced Materials and Structures (AMS), Esch-sur-Alzette, Luxembourg - Université de Strasbourg, Institut de Mécanique des Fluides et des Solides (IMFS), Strasbourg, France
- Katholieke Universiteit Leuven - Metaalkunde en Toegepaste Materiaalkunde (MTM), Leuven, Belgium
Ecole Doctorale Mathématiques, Sciences de l’Information et de l’Ingénieur
Université de Strasbourg – Centre de Recherche Public Henri Tudor –
Katholieke Universiteit Leuven
THÈSE
Présentée pour obtenir le grade de
Docteur de l’Université de Strasbourg
Discipline: Science des Matériaux
Impact of texturing on sliding wear
behaviour of UHMWPE
Présentée et soutenue publiquement par
Fatima EDDOUMY
le 29 février 2012 à Strasbourg
________________________________________________________________________________ Jury composé de :
Pierre PONTHIAUX Professeur Rapporteur (Ecole Centrale Paris) Roland SEGUELA Directeur de Recherche Rapporteur (INSA de Lyon)
René MULLER Professeur Directeur de thèse (Université de Strasbourg)
Jean-Pierre CELIS Professeur Co-directeur de thèse (Katholieke Universiteit Leuven) Christian GAUTHIER Professeur Examinateur (Université de Strasbourg)
David RUCH Docteur (HDR) Examinateur (CRP Henri Tudor)
- Centre de Recherche Public Henri Tudor, Advanced Materials and Structures (AMS), Esch-sur-Alzette, Luxembourg - Université de Strasbourg, Institut de Mécanique des Fluides et des Solides, (IMFS), Strasbourg, France
- Katholieke Universiteit Leuven - Metaalkunde en Toegepaste Materiaalkunde (MTM), Leuven, Belgium
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A mon père, Thami
A ma soeur, Jamila
A mes frères, Fouad et Marouane
CHAPTER I INTRODUCTION ... 10
I.1 Background ... 11
I.2 Problem statement ... 11
I.3 Research objectives and methodology ... 12
I.4 Manuscript description ... 13
I.5 References ... 14
CHAPTER II LITERATURE REVIEW ... 15
II.1 General features of polyethylene ... 16
II.1.1 Chemical and physical properties ... 16
II.1.2 Mechanical properties ... 20
II.1.3 Deformation mechanisms ... 22
II.1.4 Biomaterial application ... 25
II.2 Degradation mechanisms of UHMWPE ... 26
II.2.1 Degradation mechanisms by oxidation ... 26
II.2.2 Degradation mechanisms by wear ... 28
II.3 Improving wear resistance of UHMWPE ... 31
II.3.1 Cross-linking by irradiation ... 31
II.3.2 Addition of anti-oxidants ... 35
II.3.3 Modification of the initial microstructure ... 36
II.4 Conclusions ... 43
III.1 Preparation of UHMWPE ... 50
III.1.1 Consolidation by compression-moulding ... 50
III.1.2 Texturing by uni-axial stretching ... 51
III.2 Characterization of UHMWPE ... 54
III.2.1 Chemical analysis by FTIR ... 54
III.2.2 Structural characterization by SEM, XRD and DSC ... 56
III.2.3 Topographical characterization by WLI ... 62
III.2.4 Viscoelastic characterization by DMA ... 65
III.2.5 Tribological characterization by reciprocating sliding ... 66
III.3 Conclusions ... 69
III.4 References ... 70
CHAPTER IV NON-TEXTURED AND TEXTURED UHMWPE: CHARACTERISTICS AND PERFORMANCE UNDER RECIPROCATING SLIDING EXPERIMENTS ... 71
IV.1 Impact of solid-state deformation on the physical and mechanical properties of UHMWPE ... 72
IV.1.1 Solid-state deformation of UHMWPE ... 72
IV.1.2 Evolution of structural properties with WAXS, SAXS and SEM ... 74
IV.1.3 Evolution of topographical properties with WLI ... 85
IV.1.4 Evolution of viscoelastic properties with DMA ... 86
IV.2 Set-up of the tribological investigation ... 89
IV.2.1 Effect of the normal load on friction ... 89
IV.2.2 Effect of the initial roughness of UHMWPE on friction ... 91
IV.2.3 Effect of the initial deformation level of UHMWPE on friction ... 94
IV.3 Sliding performance of non-textured and textured UHMWPE ... 96
IV.3.3 Analysis of wear volume by WLI ... 107
IV.4 Relationship between texturing and wear ... 111
IV.5 Conclusions ... 117
IV.6 References ... 118
CHAPTER V GENERAL CONCLUSIONS AND PROSPECTS ... 121
V.1 General conclusions ... 122
V.2 Prospects ... 123
ABSTRACT - RÉSUMÉ ... 127
Abstract ... 128
I.1 Background
For applications that require the use components in relative motion, the quantification of wear volume and the identification of the underlying mechanisms are essential to predict the lifetime of the components, and hence, to improve their design. In the case of total hip arthroplasty, wear particles are released during movement between the metal or ceramic femoral head and ultra-high molecular weight polyethylene (UHMWPE) cup. The generated debris cause osteolysis phenomena, and in extreme cases, produce a loosening of the prosthesis that can lead to its failure [1, 2]. In these extremes cases, a medical revision is required, which is an additional burden for the patient and represents an additional cost for healthcare organizations. Therefore, it is of high interest to found ways to increase the durability of such prosthesis. It has been shown that wear mechanisms depend on the design of the prosthesis, the mechanical and chemical properties of the UHMWPE, the mechanical properties of the counterbody, and age / activities of the patient [3, 4]. The present research work is focused on the properties of UHMWPE. Numerous vitro and in-vivo studies investigated the effect of chemical structure of UHMWPE on its wear resistance under cyclic loading conditions. The results of these previous works show that irradiation-induced cross-linking considerably improves wear resistance. For example, it was shown that cross-linked UHMWPE treated by gamma irradiation exhibited a reduction by about 40-50% of the wear volume compared to conventional UHMWPE [5]. However, due to the new cross-linked molecular network, cross-linking causes a drastic decrease of tensile strength and toughness, which is not desirable [6, 7].
I.2 Problem statement
A new processing of UHMWPE is developed in this work. This method consists of stretching the material in the solid state to induce a permanent orientation of the macromolecular chains. This process is usually called texturing process and can be seen i) as an alternative to cross-linking, or ii) as a cross-linking pre-treatment. Under certain conditions, texturing may significantly increase the wear resistance of UHMWPE without altering the chemical properties [8, 9]. Texturing is also
known to increase tensile strength and toughness [10-12] prior to the irradiation cross-linking procedure. Notwithstanding that the texturing process should not decrease the wear resistance of UHMWPE prior to cross-linking. Whatever, the selected strategy, it is of fundamental interest to study the impact of the initial chain orientation state on the wear behaviour of UHMWPE. To date, only a limited number of works deals with this topic [8, 12, 13], and it is unclear whether texturing has a positive or negative effect on the sliding wear resistance of UHMWPE.
I.3 Research objectives and methodology
The main aim of this study is to investigate the influence of an initial chain orientation state of UHMWPE on the sliding wear behaviour, and to determine the underlying mechanisms. Texturing procedures were conducted by uni-axial tensile deformation procedures at 30 °C. We first analyzed the impact of the tensile deformation level on the structural, topographical and viscoelastic properties of the material. The attention is particularly focused on the evolution of the microstructure, index of crystallinity, long spacing, lamellae thickness, relaxation processes, and roughness with axial strain. Then, reciprocating sliding wear experiments were performed (ball-on-plate contact configuration, use of a corundum balls as counterbody). To this end, we developed a specific testing procedure based on the applied load, the initial UHMWPE surface state, and an energetic approach to assess with precision the sliding / wear behaviour of the material. The impact of different tensile deformation levels was studied on the friction behaviour of UHMWPE to verify whether the texture state has an impact or not on the sliding behaviour. Last, we assessed the topographical, morphological and chemical (oxidation) aspects of non-textured and the textured UHMWPE before and after sliding testing to identify the wear mechanisms active on these materials. As characterization tools, we used Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), small- and wide-angle X-ray scattering (SAXS/WAXS), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and white light interferometry (WLI).
I.4 Manuscript description
This thesis manuscript consists of four parts:
- Chapter 2 is a literature review on the general properties of polyethylene, degradation mechanisms of UHMWPE, and challenge to improve the durability of UHMWPE.
- Chapter 3 provides information about the material preparation and the characterization tools used in this research work to characterize chemical, microstructural, topographical, bulk and surface mechanical properties of the tested materials.
- Chapter 4 is dedicated to the identification of texturing mechanisms, the setting-up of the sliding testing, the study of sliding behaviour of non-textured and textured UHMWPE, including the identification of sliding / wear mechanisms.
I.5 References
[1] P. Schaaff, The role of fretting damage in total hip arthroplasty with modular design hip joints -evaluation of retrieval studies and experimental simulation methods, J Appl Biomater Biomech, 2(3) (2004).
[2] A. Wang, C. Stark, J.H. Dumbleton, Role of cyclic plastic deformation in the wear of UHMWPE acetabular cups, Journal of Biomedical Materials Research, 29 (1995) 619-626.
[3] S. Affatato, W. Leardini, M. Zavalloni, Hip Joint Simulators: State of the Art, in: Bioceramics and Alternative Bearings in Joint Arthroplasty, 2006, pp. 171-180.
[4] S.P. Ho, R.W. Carpick, T. Boland, M. LaBerge, Nanotribology of CoCr-UHMWPE TJR prosthesis using atomic force microscopy, Wear, 253 (2002) 1145-1155.
[5] J.M. Martell, J.J. Verner, S.J. Incavo, Clinical performance of a highly cross-linked polyethylene at two years in total hip arthroplasty: a randomized prospective trial, The Journal of Arthroplasty, 18 (2003) 55-59. [6] O.K. Muratoglu, D.O. O'Connor, C.R. Bragdon, J. Delaney, M. Jasty, W.H. Harris, E. Merrill, P. Venugopalan, Gradient crosslinking of UHMWPE using irradiation in molten state for total joint arthroplasty, Biomaterials, 23 (2002) 717-724.
[7] M.C. Sobieraj, S.M. Kurtz, A. Wang, M.M. Manley, C.M. Rimnac, Notched stress-strain behavior of a conventional and a sequentially annealed highly crosslinked UHMWPE, Biomaterials, 29 (2008) 4575-4583. [8] D.S. Li, H. Garmestani, S. Ahzi, M. Khaleel, D. Ruch, Microstructure Design to Improve Wear Resistance in Bioimplant UHMWPE Materials, Journal of Engineering Materials and Technology, 131 (2009) 041211.
[9] D. Li, H. Garmestani, A. Chu, H. Ahzi, G. Alapati, M. Khatonabadi, O. Es-Said, M. Siniawski, L. Matrisciano, S. Ahzi, Wear resistance and microstructure in annealed ultra high molecular weight polyethylenes, Polymer Science Series A, 50 (2008) 533-537.
[10] F. Addiego, O. Buchheit, D. Ruch, S. Ahzi, A. Dahoun, Does Texturing of UHMWPE Increase Strength and Toughness?: A Pilot Study, Clinical Orthopaedics and Related Research®, 469 (2011) 2318-2326.
[11] A. Galeski, Strength and toughness of crystalline polymer systems, Progress in Polymer Science, 28 (2003) 1643-1699.
[12] S.M. Kurtz, D. Mazzucco, C.M. Rimnac, D. Schroeder, Anisotropy and oxidative resistance of highly crosslinked UHMWPE after deformation processing by solid-state ram extrusion, Biomaterials, 27 (2006) 24-34. [13] H. Marrs, D.C. Barton, C. Doyle, R.A. Jones, E.L.V. Lewis, I.M. Ward, J. Fisher, The effect of molecular orientation and acetylene-enhanced crosslinking on the wear of UHMWPE in total artificial joints, Journal of Materials Science: Materials in Medicine, 12 (2001) 621-628.
Literature review
The objectives of this chapter is to provide an overview about the general properties of polyethylene and to highlight the scientific challenges of ultra-high molecular weight polyethylene (UHMWPE) used as biomaterial. Attention is first focused on the multiscale structure, the mechanical properties, the deformation mechanisms, and the application of polyethylene as a component of medical prosthesis. Then, relevant information about the chemical and mechanical durability of UHMWPE is reported. Last, we describe how the durability of UHMWPE can be improved by cross-linking, addition of anti-oxidant, modification of the crystallinity and texturing. Finally the scientific challenges arising from these strategies are listed
II.1 General features of polyethylene
II.1.1 Chemical and physical properties
Polyethylene is a material obtained by polymerization of ethylene C2H4 (Figure II-1) leading to
macromolecules composed of the repeating monomer unit -(CH2-CH2)n-. The molecular
architecture of this material can be controlled through the polymerization process in terms of chain length, distribution of chain length and branching type and length.
Figure II-1: Polymerization of polyethylene
Polyethylene grades are generally classified into three categories: low density polyethylene (LDPE) with long branches, linear low density polyethylene (LLDPE) with short branches and high density polyethylene (HDPE) composed of linear chains. Ultra-high molecular weight polyethylene (UHMWPE) is a particular case of HDPE characterized by very long chains. HDPE has a molecular weight that is typically comprised between 50,000 and 250,000 g/mol, while that of UHMWPE is of about 3 to 6 million g/mol. UHMWPE is synthesized by a catalytic polymerization process named Ziegler-Natta catalysis with the use of a titanium chloride catalyst [1].
This polymerization process is carried out in a solvent which allows the reaction for mass and heat transfer; UHMWPE is in powder state after synthesis. One of the main producers of this product is the company Ticona that kindly provided the UHMWPE grade used for this study in the form of a powder (GUR1050). The designation GUR means Granular Ruhrchemie. The first digit (1) corresponds to the loose bulk density of the resin, that, is the weight measurement of a fixed volume of loose, unconsolidated powder density, the second number corresponds to the presence (1) or absence of calcium (0), the third digit (5) corresponds to the molecular weight (million g/mol) and last digit is an internal code of Ticona [2].
As semi-crystalline polymer, UHMWPE contains crystalline lamellae that have a planar shape and are formed by chain folding (Figure II-2). The typical thickness of UHMWPE crystalline lamellae is of about 15-25 nm, while for HDPE it is comprised between 5 and 15 nm. Lamellae are interconnected by a few chains, called tie molecules that pass from one lamella through a small amorphous region, to another lamella (Figure II-2).
Figure II-2: Morphological features of UHMWPE [2]
Table II-1 shows the main characteristics of three polyethylene types in terms of density, molecular weight, and weight fraction of crystalline phase [1].
Table II-1: Typical physical properties of LDPE, HDPE and UHMWPE [1]
Density
(g / m 3) Molecular weight (g / mol) crystallinity (%) LDPE 0.910 – 0.940 10,000 40
HDPE 0.952 – 0.965 50,000 – 250,000 60-75
UHMWPE 0.930 – 0.945 3,000 000 – 6,000000 50-60
Whatever the chain architecture, polyethylene is composed by an amorphous phase and a crystalline phase with variables proportions. In the amorphous phase, the amorphous chains have a random arrangement and are entangled. Among the three polyethylenes reported in the Table II-1, UHMWPE is characterized by the highest density of entanglements. Regarding the crystalline phase, molecular chains have a zigzag conformation induced by the crystallization process (Figure II-3).
Figure II-3: Schematic conformation of macromolecular chains of polyethylene; a) random conformation, b) Zigzag conformation [3]
At the nanoscale (1 nm), the most stable crystalline lattice of polyethylene is the orthorhombic system characterized by the following parameters (measured by Bunn in 1939): a = 0.740 nm, b = 0.493 nm and c = 0.253 nm (Figure II-4).
a) b)
Figure II-4: General aspects of crystalline lamellae of polyethylene, a) two successive crystalline lamellae, and b) chain configuration in a lamella [3]
The thickness of crystalline lamellae (Lc) is the length of the chain segment between two successive
folds. Lc is typically a few tens of monomer units (about 15 -25 nm for UHMWPE). Between two
crystalline lamellae there is an amorphous phase with a thickness (La) which is comparable to that
of crystallites. A typical characteristic of this specific morphology is the long spacing Lp that is
given by:
ܮൌܮ ܮ II-1
Note that the radial arrangement of stacks of crystalline lamellae and amorphous layers is called spherulites which have a diameter ranging between 5 and 100 µm. In the case of UHMWPE, the high viscosity of this material and the important density of entanglements do not enable the formation of spherulites. In this case, the supercrystalline structure is an aggregate of intertwined crystalline lamellae (Figure II-5).
Figure II-5: Typical supercrystalline structure of UHMWPE [4]
II.1.2 Mechanical properties
We qualitatively describe here the mechanical properties of polyethylene through the description of tensile behaviour which provides information about the viscoelastic, and viscoplastic properties of this polymer, while Table II-2 reports some general characteristics of the tensile behaviour.
During a tensile test (Figure II-6), the behaviour of polyethylene is characterized by two stages: i) the viscoelastic stage up to the true strain of about 0.1, and then the viscoplastic stage up to the breakup of the material. The initial slope of the stress-strain curve provides the value of the elastic modulus of the material that is comprised between 0.1 and 1.5 GPa. Elastic modulus generally decreases with increasing temperature or decreasing strain rate. Moreover, elastic modulus increases with the degree of crystallinity and the entanglements density of the amorphous phase. The transition between the two deformation stages is characterized by a knee-like shape where the stress gradually stabilizes. The stress at the transition between these two stages is called the yield stress and is generally comprised between 10 MPa and 30 MPa depending on crystalline lamellae thickness, temperature and strain rate. In the final part of viscoplastic stage, a marked increase of stress with strain is noted: it is called strain-hardening process. This latter is much more marked for UHMWPE than for the other polyethylene categories due to the high density of entanglement and tie molecules in UHMWPE that causes a high resistance to deformation during stretching [5].
Figure II-6: True-stress strain behaviour in uniaxial tension (room temperature, dε/dt = 30 mm/min) for two grades of UHMWPE, in comparison to HDPE [5].
Table II-2 summarizes the physical and mechanical properties generally reported for HDPE and UHMWPE.
Table II-2: Physical and mechanical properties of HDPE and UHMWPE [2]
UHMWPE has a higher energetic toughness (ability to absorb energy) owing to its very high molecular weight, crystallinity, and chain entanglement density.
Moreover, UHMWPE is significantly more abrasion resistant and wear resistant than HDPE. These properties make UHMWPE suitable for bearing surface application requiring a polymeric material as artificial joints for prosthesis. For example, some hip simulator procedures show that the volumetric wear rate of UHMWPE is 4.3 times lower than that of HDPE (Figure II-7).
Figure II-7: Wear rate of HDPE and UHMWPE in multidirectional hip simulator [2, 6]
II.1.3 Deformation mechanisms
We describe here the deformation mechanisms occurring during the tensile deformation of polyethylene, which are at the origin of the texturing. Indeed, the stretching procedure of polyethylene implies first reversible mechanisms and then non-reversible mechanisms that lead to permanent deformation, and hence a texture. Generally, tension induces a gradual chain orientation process in amorphous layers and rotation, shear and fragmentation of crystalline lamellae [7, 8]. The elementary deformation mechanisms are reported below [3].
When stress is applied to the material, deformation occurs first in the amorphous phase that deforms by (Figure II-8):
interlamellar shear, interlamellar separation, or interlamellar compression
The occurrence of these mechanisms depends on the orientation of the lamellae towards the stress. In particular, lamellae oriented perpendicular to tensile direction are subjected to interlamellar separation, lamellae oriented along tensile direction are subjected to interlamellae compression, while the lamellae oriented at 45° toward the tensile direction are subjected to interlamellar shear mechanism. For all the other configurations, the amorphous phase is subjected to a combination of these three elementary deformation mechanisms.
Figure II-8: Mechanisms of deformation of the amorphous phase in semi-crystalline polymers; a) undeformed state, b) interlamellar slip and c) interlamellar separation [3].
From a certain strain level, the stresses are transmitted from the amorphous chains to the crystalline ones via the tie molecules. This transition corresponds to the yield point. Then, the deformation appears within the crystalline phases and is characterized by intralamellar slip mechanisms parallel (chain slip) or perpendicular (transverse slip) to chain axis (Figure II-9). Note that chain slip requires less energy than transverse slip. The combination of all these elementary deformation mechanisms leads to a gradual orientation of the chain along the tensile direction and in the same time to a fragmentation of crystalline lamellae into small blocks (Figure II-10). The resulting microstructure is called micro-fibrillar morphology.
Figure II-9: The process of intralamellar sliding a) parallel b) perpendicular to the chains of crystalline lamellae [3].
Figure II-10: Schematic representation of the mechanisms of fragmentation of crystalline lamellae; a) undeformed state, b) interlamellar separation and shear, c) intralamellar slip mechanisms and
fragmentation of the lamellae, d) resulting microfibrillar morphology [9].
As regards UHMWPE, the intralamellar slip mechanisms are partially activated. Indeed, the high lamellae thickness, associated with an important number of entanglements in the amorphous phase, leads to a restriction of lamellae fragmentation. In particular, a bending mechanism of lamellae is active instead of a fragmentation which generates the formation of chevron-like lamellae (Figure II-11) [8].
Figure II-11: Example of chevron type-patterns lamellae in UHMWPE strained up to a true axial strain of 1.3 [4]
II.1.4 Biomaterial application
Each year more than 800,000 total hip arthroplasty (THA) are performed worldwide [10]. THA enables the replacement of the two articular surfaces of a hip joint. The prosthesis consists of a rod implanted in the femur in contact with a prosthetic cup implanted in the hip. The materials used for the design of this hip replacement must address the following specifications:
to be biocompatible, the ability to adapt to the biological media [11]. As this environment is generally very corrosive, a good corrosion resistance is also required;
to have a high mechanical durability due to high cyclic stress during the movement [12]; to have a high wear resistance and low friction to prevent the release of particles (wear
debris) that may negatively affect the mechanical and the biological properties in the body (inflammatory reactions that lead to the release of osteolytic substances or loosening of the implant was observed by the isolation of macrophages containing wear debris). Currently, the debris appears as the main factor that dictates the lifetime of the prosthesis [13].
Depending on the element of the prosthesis, different materials are used (Figure II-12): the majority of the implanted cup are made of UHMWPE, some cups are on ceramic;
materials used in the stem are made of titanium alloy (Ti6Al4V) or cobalt-chromium alloy (Co-Cr-Mo) or stainless steel (316L);
Figure II-12: Components of a total hip replacement [13]
The use of UHMWPE in total joint replacement prosthesis was introduced in the 1960s. Initially, polyetrafluoroethylene (PTFE) was used as a bearing material because of its low friction coefficient against metallic counterface but it exhibited high wear rate and hence important inflammatory reactions of the human body. Therefore, PTFE was replaced by UHMWPE, a material that has a lower wear rate than PTFE. It is foreseen that the demand of THA will increase within the next years, for both young and old patients. However, despite its good wear resistance, the durability of UHMWPE is not high enough and gives rise to an important number of prosthesis revisions [14, 15]. One of the scientific challenges is to reduce the revision rate of the prosthesis and hence to avoid additional burden for the patients and further cost for the health system.
II.2 Degradation mechanisms of UHMWPE
II.2.1 Degradation mechanisms by oxidation
During compression-moulding, it is quite difficult to avoid oxidation of the polymer, even when compression-molding is performed in the presence of an inert gas such as nitrogen [16]. In particular, hydroperoxides functions (ROOH) are present in the UHMWPE. These functions are thermally unstable since they can transform into hydroperoxides macroradicals (ROO.). The latter can react with oxygen which leads to a series of reactions, which involves an oxidation process.
The extent of this oxidative process depends on the number of radicals and the amount of oxygen in the polymer. Due to this oxidation, there is a significant decrease in molecular weight which results in lower mechanical properties and possible delamination mechanisms. In the case of THA, a sterilization procedure by gamma rays irradiation is performed on the polyethylene component to kill bacteria that could affect the patient. One drawback of this procedure is that free-macroradicals are formed and hence, react with oxygen given rise to a marked degradation [17, 18]. For both compression-molding and sterilization procedures, the polymer is exposed to oxygen and therefore to degradation. This degradation process is schematically shown in Figure II-13 by the cycle of Bolland.
Figure II-13: Bolland’s cycle [16]
In these reactions, the initial radical reacts with oxygen to form a peroxyl radical (reaction 1). After this step, the peroxyl radical may react with a neighbouring polymer chain to form a hydroperoxide (Reaction 2). Such a hydroperoxide is thermally unstable and decomposes photochemically into an alkoxide radical PO. and a hydroxyl radical OH. Alkoxide radicals generated can evolve into ketones (-C = O), aldehydes (-CH = O), and alcohols (-C-OH) (Reaction 3). The aldehyde can oxidize very rapidly into acid (-COOH). The chemical functions present can be identified by FTIR.
II.2.2 Degradation mechanisms by wear
Abrasive wear, adhesive wear and fatigue wear have been identified as the three driving mechanisms which induce surface damage and deterioration of UHMWPE used as a bearing surface [19-21].
Abrasive wear produces surface topographies characterized by long grooves as the result of plastic deformation and rupture at the micron and sub-micron scales (microcutting). One can differentiate abrasive wear by two bodies or by three bodies. The two body abrasive wear occurs when a hard rough surface slides against a softer surface. The wear rate depends then on surface roughness of hard material and its relative hardness compared to that of the counterface material. In total joint replacements this form of wear is decreased by polishing the metallic counterface surface (for example Co-Cr in the case of THA). As regards three body abrasive wear, it occurs when hard particles are abraded onto two sliding surfaces. Generally hard abrading particles will be embedded in the softer material. In clinical cases, abrasive wear occurs when bone cement particles or metallic particles are present between the two sliding surfaces of a hip implant [22, 23].
Regarding adhesive wear, it is generally due to unwanted displacement and attachment of wear particles from one surface to another. This mechanism can be characterized by the release of wear particles and material transfer, as well as cohesive forces which hold the two surfaces together. This kind of wear is indicative of high frictional forces and is generally more prevalent between similar materials (i.e., metal against metal) [24].
Last, fatigue wear (or delaminating wear) mainly depends on the load and the number of cycles. The general sequence of events is as follows: the two sliding surfaces come into contact, asperity against asperity. The asperities of the softer material are deformed and some of them are fractured through repetitive loading resulting eventually in the formation of a smooth surface. Consequently there is no longer an asperity/asperity contact but an asperity/plane contact. Then, accumulation of
plastic shear deformation from repetitive loads takes place on the softer material due to the elongation of the hard asperities. Afterwards, crack nucleation takes place below the surface. Finally, these cracks propagate along the joining surface, coalesce, and propagate to the surface. This mechanism generates long and thin wear debris [24].
The study of wear in total joint replacement has taken two major routes, in vivo and in vitro [25]. For in vivo studies [26-29], attention is focused on the characterization of the wear features that caused the failure of prosthesis by studying the two sliding surfaces. In the case of THA, clinicians generally investigate the wear tracks of UHMWPE bearing component and the characteristics of the wear debris accumulated in the periprosthetic tissue. Regarding in vitro studies [28, 30-34], attention is devoted to the laboratory simulation of wear. Such studies are generally conducted to validate the formulation of a new material or a new prosthesis design prior to in vivo studies. From the in vivo studies [26-29], it has been found that most prosthesis failures that required a revision were due to osteolysis phenomena. It was proved that the latter were linked to the reaction of macrophage toward the wear debris of the UHMWPE bearing component. The examination of explanted UHMWPE cup revealed the presence of large craters at the origin of the release of wear particles. Primarily, it was believed that these craters were due to abrasive wear mechanisms from the hard acrylic particles (bone cement). Indeed, the area surrounding the craters was generally scratched and some cement particles were found embedded in the polyethylene. Latter, it was found that craters were actually generated by the cyclic loading stresses that cause local plasticity mechanisms and fracture / fatigue crack propagation mechanisms. Local plasticity mechanisms imply first the formation of fibrils and then the breaking up of the latter, which leads to the formation of wear particles. A low fracture and/or crack propagation resistance leads to delamination and formation of large wear particles of the UHMWPE component. It is important to mention, that the smallest wear debris, that are considered as nanometric, leads to the most marked body reaction. The following Table II-3 reports the two basic wear mechanisms (fatigue wear and surface wear as shown in Figure II-14) occurring in hip prosthesis, but also in knee prosthesis.
Table II-3: Two basic wear mechanisms (fatigue and surface wear)
Type of wear
mechanisms Type of implant Basic mechanisms description References
Surface wear In hip and knee implants (same proportion)
Local formation of polymer fibril due large deformation conditions,
rupture of the fibrils and formation of wear particles, this
phenomenon is due to the resistance of the polymer network
to deformation (high strength is required)
[32, 35, 36]
Fatigue wear
Mainly in knee implant, less reported case for
hip implant
Local formation of cracks below the surface, propagation and coalescence of the cracks, this phenomenon is due to a subtle combination of fracture and crack
growth mechanisms
[37-39]
a) b)
Figure II-14: a) Fatigue wear mechanism (4 reported cases corresponding to 4 different designs) [40], and b) surface wear mechanism [20, 41]
II.3 Improving wear resistance of UHMWPE
This part is focused on four aspects which dictate the chemical and mechanical durability of UHMWPE, namely oxidation, cross-linking, crystallinity and texturing. Oxidation phenomena are active during the polyethylene processing and in the human body, and significantly change the physical properties of the polymer network inducing a decrease of ductility. Such a decrease of ductility is not desirable for the wear mechanisms of UHMWPE. It is of fundamental importance to find a strategy to reduce oxidation. Regarding cross-linking, it is a method that is now well-accepted to reduce wear of UHMWPE due to an increase of the resistance to deformation of the molecular network. But, at the same time, the chemical stability, toughness and tensile strength decrease what is not suitable. To date, avoiding such drawbacks of cross-linking appears to be a scientific challenge. Last, some pioneer works [42-46] showed that acting on the initial crystallinity and orientation state of the chains (texture) can have positive effects on the mechanical strength and resistance to fracture of UHMWPE. Further research works are required regarding this last strategy.
II.3.1 Cross-linking by irradiation
Improving the wear resistance of UHMWPE through an irradiation step leads to an increase of the degree of cross-linking of the material. Cross-linking systematically occurs at high doses of gamma or electron radiation, while sterilization procedure of the cup is conducted at low irradiation doses. Note that irradiation changes the chemical structure of the polymer because of chain scission mechanisms (C-C and C-H bonds) and produces free radicals (Figure II-15). Two phenomena can then occur, namely i) a reaction between these radicals and oxygen of the environment (oxidation, Figure II-13), or ii) a reaction between radicals to form a covalent bond between adjacent chains inducing the formation of a 3D network (Figure II-16).
Figure II-15: The process of irradiation [16]
Figure II-16: The phenomenon of cross-linking [16]
Different authors [43, 47-53] studied the influence of irradiation and environment conditions on the physical and mechanical properties of UHMWPE. They compared for example a non-irradiated and an irradiated material (obtained by gamma irradiation) at different dose levels and found that the higher the irradiation dose was, the greater the wear resistance (Figure II-17). This was explained by a higher resistance of the molecular network to deformation, which reduces the formation of fibrils and hence of wear debris. However, other mechanical properties such as crack propagation resistance and fracture resistance decrease with increasing radiation dose. This phenomenon is explained by a lower activation of plasticity mechanisms in the case of cross-linked UHMWPE compared to the reference material (non cross-linked).
Figure II-17: Wear rate as a function of radiation dose [54]
Laurent et al.[55] characterized a UHMWPE with a high degree of cross-linking. Two samples were subjected to two different treatments, namely i) irradiation by electron beam (100 kGy) followed by annealing below the melting point and plasma sterilization (gas), and ii) sterilization by gamma irradiation. For the first sample, the absence of free radicals leads to an improvement of oxidation resistance, which is suitable regarding the stability of the mechanical properties.
Costa et al. [17] studied the initiation of oxidation by focusing on the chemical reactions created during oxidation. The aim of their work was to analyze the irradiated UHMWPE (electron beam) using different doses and in different atmospheres (vacuum / air / pure oxygen). They found an increased concentration of hydroperoxides (ROOH) as a function of irradiation dose at room temperature and in air. This increase seems to be due to the reaction of secondary alkyl macroradicals formed during irradiation with oxygen (Figure II-15). This chemical modification of UHMWPE leads to oxidation which is one of the main weak points in UHMWPE.
Dalborg et al. [56] analyzed the spatial distribution of oxidation using different characterization techniques. They studied the distribution of sites of oxidation by comparing light microscopy (staining of hydroperoxides by SO2 (gas) and HCl (35% solution) followed by heat treatment (95
°C for 24 hours with SO2 and 95 °C for 4 hours with HCl)), chemiluminescence and FTIR. They
just below the surface. They also observed an oxidation in the mass that is associated with poor consolidation of the material.
Lee et al. [57] studied in vivo the impact of irradiation (oxidation and cross-linking) on the wear resistance of UHMWPE. Irradiation increases the cross-linking density and the degree of crystallinity because the shorter chains have more mobility and can cross-link or recrystallize more easily than the initial macromolecules. At in-vivo level, oxidation and cross-linking of polyethylene are in competition to capture free radicals once created. Indeed, the authors found that the degree of oxidation (obtained by FTIR) increases with time and the degree of cross-linking (assessed by gel fraction measurements) decreases with time. They also found a decrease in wear resistance (wear test performed by unidirectional pin (UHMWPE)-on-disc (316L)) which is accompanied by the formation of a white area and a weakening of the material. The difference in behaviour between the degree of crystallinity and degree of cross-linking would imply that free radicals are still present in the material. These free radicals generate an intermolecular and intra-molecular decomposition resulting from a splitting of chains in the amorphous phase. These chains are rearranged and the degree of crystallinity (measured by DSC) increases. These changes in cross-linking, crystallinity, and oxidation obviously impact the mechanical properties. In this latter study, polyethylene wear has been associated with the existence of white strip of brittle fracture, a high degree of oxidation and a low percentage of cross-linking of the material.
The irradiation step is a source of oxidation, and therefore weakens the material, but many studies [17, 44, 45, 51-53, 57-63] have shown that irradiation is essential to obtain good tribological properties in terms of surface wear resistance. To limit the unwanted effects of cross-linking, the irradiation procedure must be associated with pre- or post-treatments of the material. To reduce or eliminate sources of free radical oxidation, the irradiated UHMWPE materials can be heated above (150 °C) or below (130 ° C) their melting temperature. Some authors, as Kurtz et al. [43, 53, 64] have shown that above the melting point (during a remelting step), the crystalline phase is destroyed and the amount of free radicals decreases because they combine to each other but the degree of crystallinity also decreases, which directly impacts the mechanical properties. Below the melting point (during an annealing step), the degree of crystallinity is not affected but free radicals are not totally eliminated, what causes oxidation. A compromise has in general to be found regarding the stabilization temperature or other treatments that are required.
These studies showed that a chemical modification of UHMWPE leads to oxidation which is one of the main weak points in hip prosthesis.
II.3.2 Addition of anti-oxidants
Some works deal with the addition of stabilizers to decrease or eliminate the oxidation process [17, 51, 65]. Oral et al. [51] introduced E-Vitamin into UHMWPE prior to irradiation. After irradiation, the splitting of the OH bond in the molecule of E-Vitamin leads to the formation of O radical and the hydrogen atom will react with the macroradical of UHMWPE as shown in Figure II-18. Oral et al. [51] indicated that less than 0.3 wt. % of E-Vitamin is recommended to preserve the wear resistance.
Figure II-18: Reaction between UHMWPE and vitamin E [51]
As already mentioned, the cross-linking density is an indicator of wear resistance, and it was found that cross-linking improves wear resistance. However, the cross-linking density decreases with increasing the concentration of E-vitamin. Further studies are needed to optimize the formulation and the processing of E-Vitamin / UHMWPE materials in terms of oxidation stability and surface wear resistance. It is to be highlighted that no stabilizer can be used in hip replacements according to ASTM F 648 [66]. Nevertheless, Ticona Company already supplies UHMWPE with E-vitamin for medical applications.
II.3.3 Modification of the initial microstructure
The crystallinity index is well-known to dictate most of the bulk mechanical properties of a semi-crystalline polymer as Young’s modulus, yield stress, plasticity mechanisms and hence surface mechanical properties.
Karuppiah and al. [67] investigated the impact of crystallinity on the friction and wear of UHMWPE. In this context, two UHMWPE with two different degrees of crystallinity were analyzed. The degree of crystallinity was controlled by the processing of the material. In particular, the material was first slowly cooled in an air flow to promote crystal growth (high crystallinity), and in a second step the material was quenched in nitrogen to limit crystal growth (low crystallinity). The degree of crystallinity was determined by DSC: it is higher in the first case (55.1 %) than in the second case (45.6 %) and no significant variation between the skin and the core of the material was found. To analyze the influence of crystallinity on the surface mechanical behaviour of UHMWPE, different analysis techniques were used, namely:
i) The analysis of the topography by AFM showed a larger strip size when cooling is slower, and so, when the degree of crystallinity is higher (Figure II-19).
Figure II-19: Topography of samples at high (left) and low (right) index of crystallinity [67]
ii) The analysis by means of a micro-scratch tester revealed that the sample with a high degree of crystallinity has a higher scratch resistance (lower depth and width of the scratch groove)
and a higher friction force than the sample with a low degree of crystallinity. The same tendency regarding the scratch groove depth was obtained by AFM (Figure II-20).
Figure II-20: Scratch depth in two different areas of the sample with a high degree of crystallinity (region 1, the blade shape of the crystalline phase is better defined than for region 2) [67]
The above points i) and ii) show that the lamellar structure affects the surface mechanical properties of the material.
UHMWPE has been recrystallized with different cooling conditions for the purpose of enhancing cross-linking extent of the polymer after Ȗ-irradiation by Kang and Nho [68]. It was shown that the crystallinity of the irradiated samples increased with the irradiation dose. The irradiated UHMWPE after quenching had a lower sliding wear rate than the irradiated UHMWPE after recrystallization with slow cooling conditions, and the sliding wear rate of UHMWPE decreased with irradiation dose up to 250 kGy, resulting in about 40% of the wear rate of non-irradiated UHMWPE. From these experimental results, the authors expect that UHMWPE having the enhanced cross-linking after Ȗ-irradiation will be used to extend the life time of the artificial joints.
Tribology tests [69] (reciprocating sliding, under dry conditions, done with a ball-on-plate configuration, spherical Si3N4 ball with a radius of ~ 1.2mm) showed that the lower the degree of
crystallinity, the higher the friction force (Figure II-21). Ho and al. [69] suggested that this increase in friction with decreasing degree of crystallinity is due to the consequent decrease of the modulus of elasticity. Indeed, in semi-crystalline polymers, the crystalline phase is more rigid than the amorphous phase, what implies that the elastic modulus increases with increasing degree of crystallinity.
Figure II-21: Friction force as a function of normal force with a tribometer (reciprocating sliding, under dry conditions, done in a ball-on-plate configuration, spherical Si3N4 ball
with a radius of ~ 1.2 mm) [67]
In addition, some sliding tests were conducted using the same configuration than the tribological tests (reciprocating sliding, under dry conditions, done in a ball-on-plate configuration, spherical Si3N4 ball with a radius of ~ 1.2 mm) and confirmed the previous tendency regarding the impact of
crystallinity on frictional force.
The mechanical properties measured by nanoindentation (hardness and elastic modulus) showed that the higher the crystallinity, the higher the hardness and elastic modulus. These mechanical properties are directly related to the decrease in friction force observed by tribological testing and increased scratch resistance observed by scratch testing.
Pruitt et al. [52, 70, 71] modified the microstructure of UHMWPE by hot-pressing that leads to an increase of the crystallinity. This increase enhances the elastic modulus and hardness, improving the resistance to crack propagation by fatigue, and the tensile fracture resistance, and keeping a good wear resistance. The modification of crystallinity was associated with cross-linking. Three samples were analyzed, namely one reference sample, one hot-pressed sample (300 MPa at 180 °C for one hour), and one irradiated/stabilized/hot-pressed sample (gamma irradiation, 50 kGy, stabilization at 170 °C for 4 h and 125 °C for 48 hours, hot-pressed at 500 MPa at 240 °C for one hour with a cooling procedure carried out for 1 h at room temperature without pressure). This study demonstrated that the degree of crystallinity obtained by DSC increases with the pressure force of the hot-pressing procedure compared with the reference material. As shown by USAXS measurements, when the reference material is hot-pressed, crystal-growth is favoured with respect to crystal- nucleation. The opposite behaviour is observed when the cross-linked material is hot-pressed. Indeed, cross-linking appears as an obstacle to the growth of polyethylene crystals (the mobility of the molecules is reduced by the 3D network). In both cases, the UHMWPE was characterized by a higher crystallinity and crystal thickness than the reference material. It has been demonstrated that when the size of crystalline lamellae increases, the fatigue resistance of UHMWPE increases [52]. This result was linked to the increase in yield stress that dictates the initiation of the cracks.
Ohta et al. [44-46] studied the impact of the orientation state of the UHMWPE molecules to improve the wear properties of the material. The material was first subjected to a low dose (2.0 Mrad with γ-rays) of irradiation (cross-linking procedure). Then, the orientation state was obtained by a compression process performed at the melted state (temperature of the procedure 200 °C) by means of a ball to achieve the shape of a cup (Figure II-22). Different strain levels were reached (strain calculated by comparison between the final thickness and the initial thickness). The material was cooled down to room temperature.
Figure II-22: Preparation of the UHMWPE sample by compression at molded state [46]
Ohta et al. [46] first examined the influence of the radiation dose on the cross-linking degree obtained by gel fraction. From a certain dose of irradiation (1.8 Mrad), the density of cross-links becomes more important and the orientation becomes more difficult to achieve. This threshold gives the optimum irradiation dose leaving sufficient mobility to the chains. The material properties assessed by DSC (melting temperature), XRD (crystallinity) and micro indentation (hardness) showed that an increase of the compression pressure causes an increase of the melting temperature, the index of crystallinity and the hardness. Wear tests were carried out in a tribometer plan (Co-Cr) - pin (UHMWPE) configured to perform reciprocating sliding tests. After the reciprocating sliding tests, the wear factor is improved for compressed samples by a factor of 5. The compressed samples appear harder than non-compressed samples in Vickers hardness measurements. It was found that what seems to improve the wear factor is not the crystallinity degree as a whole, but the crystal structure (molecular chains orientation) on the sample surface. The result indicates that the sliding wear rate decreases with increasing the compression strain. Moreover, it was found that the degree of crystallinity does not directly influence the wear resistance. What actually influences the sliding wear behaviour is the orientation state of the crystals imposed by the compression procedure, as shown by WAXD (Figure II-23). The increase in the degree of orientation leads to a decrease in the wear factor. However, little is known about these mechanisms.
Figure II-23: Diffraction patterns depending on the point of deformation [46]
Texturing affects mechanical properties as strength, fracture resistance and fatigue crack propagation. The strength of semi-crystalline polymers is markedly affected by the orientation state of the chains (texture) that can be imposed by a solid-state deformation process. Indeed, compared to the non-deformed state, an initially-stretched UHMWPE has an increased strength when stretched parallel to the chain direction and a decreased strength when stretched perpendicular to the chain direction [10]. Therefore, textured semi-crystalline polymers can be considered as long-nanofibre polymer composites with a high number of connections between the fibres and the matrix and a high fatigue and fracture resistance in the fibre direction due to the high strength level in this direction. These new features can be appropriate for applications that require wear resistance to uniaxial tension. Thus, as suggested by Kurtz et al. [11], the cup in THA could be designed with the polar axis aligned with the texture direction. The wall of the cup at the equator and rim is hence parallel to the texture direction and may benefit to the high strength during eccentric and rim loading conditions. Nevertheless, UHMWPE component used in artificial joints are generally submitted to multiaxial loading conditions. A detailed study about the influence on the mass mechanical behaviour of the texture of such structural components is consequently required to assess whether solid-state deformation constitutes an effective treatment of UHMWPE in joint
prosthesis. Little is known about the influence of an initial molecular alignment of UHMWPE on the wear behaviour prior to cross-linking.
Li et al. [72, 73] investigated the wear behaviour of pre-compressed UHMWPE samples with a linear reciprocating tribometer using a ball-on-flat configuration. These authors showed that from a strain level of 20 %, the wear volume of pre-compressed UHMWPE significantly increased parallel to the chain direction (Figure II-24a) and decreased perpendicularly to the chain direction (Figure II-24b). Micromechanical models are used to predict the evolution of the microstructure and the improvement in wear resistance during processing. Predicted results agree well with experimental data. These models may help the materials designer to optimize processing to achieve a better sliding wear behaviour along desired directions.
Figure II-24: Measured and predicted wear behaviour and predicted elastic modulus of UHMWPE on the plane parallel to the compression direction (a) along the direction parallel to the compression
direction, and (b) along the direction perpendicular to the compression direction [72]
However, the previous study does not provide any wear mechanism in terms of dissipated energy and topographical features. Furthermore, the sliding wear tests performed in this previous study (high track length) were not representative of the relative motion in the hip (low track length due to fretting).
II.4 Conclusions
This literature review points out that the use of UHMWPE as biomaterial in THA must address numerous specifications regarding physical properties, chemical properties, bulk and surface mechanics, and biological aspects, which represents a complex topic of research. Current scientific challenges are to increase the chemical and mechanical durability of UHMWPE to decrease the revision rate of the implants due to the failure of this material. One treatment of interest for UHMWPE is texturing since i) it does not alter the chemistry of the material, ii) it is an easy and well-controlled process, and iii) texturing can increase resistance to fracture and resistance to crack propagation and hence can decrease fatigue wear. This treatment can also be suitable to increase the mechanical strength of UHMWPE in the chain direction for particular conditions, which can be favourable. Regarding surface wear mechanisms, little is known about the influence of chain orientation on the formation of wear debris. Further researches are needed to assess whether surface texturing would be a relevant treatment of UHMWPE in the context of medical implants.
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